1. Field of the Invention
The invention pertains to electrical circuit interconnections and, more particularly, to the addition of nano-structures that facilitate thermal dissipation and electrical conductivity in microcircuits that are fabricated using conductive materials and anisotropic conductive adhesives.
2. Discussion of the Related Art
Clearly, the continuing development of microcircuits includes, among others, the objectives of: application of flexible printed circuits, increased capacity (more switching functions in smaller devices), and a host of robustness issues, such as moisture control, improved shock-resistance, and use in higher temperature applications. These issues become more crucial when printed circuits are used in environments in which they are shocked or vibrated, as in machinery or fighter planes, or when high temperature, moisture, or contamination is experienced, as in industrial corrosive and high-humidity environments and in the engine compartment vehicle. To achieve these objectives, improvements in the connections between microcircuit components and the circuit chip or printed circuit board must be made.
Conductive adhesives and anisotropic conductive adhesives (ACA) have been used regularly in microcircuit fabrication, and their composition has been well described. Isotropic conductive adhesives (ICA) (or merely “conductive adhesives”) provide uniform conductivity along all axes, and thus do not isolate adjacent conductors which are each embedded in the matrix. The distance across a gap between conductors intended to be electrically connected, and the distance between adjacent conductors intended to be electrically isolated may be of similar scale. Thus, insulating barriers are required for isolation, making bulk dispensing problematic.
These isotropic adhesive compositions consist primarily of an insulating adhesive resin carrier in which a matrix of interconnect fine particles is suspended. For the purpose of this description, such fine particles and the device and circuit board connections, including metal, metalized polymer, carbon or carbonaceous, micron or sub-micron sized shapes, including spheres, rods, tubes, conductors for heat transfer or electrical connection, printed circuit substrates and lands, and/or other regular and irregularly shaped particles and connectors, upon which nano-structures are attached or grown, are referred to as spheres. The spheres in the adhesive compositions, when squeezed under pressure during microcircuit fabrication, interconnect the components and layers of the microcircuit chip or circuit board. It is to be particularly emphasized that the nano-structures may be grown on flat surfaces of the conductor pads, the active device (chip), printed circuit substrates and connectors and are not limited to the surface of particles in dispersion within an adhesive matrix. In other words, any body on whose surface these nano-structures are grown is referred to as a sphere, regardless of its shape.
For the purpose of description, the nano-structures are drawn as columns in the figures, but they may be spikes, cylinders, tubes, hemispheres, fibers, or any other regular or irregular shape; they are referred to as nano-structures.
The adhesive compositions have several purposes including, but not limited to: providing the carrier medium for the matrix of interconnect spheres to be distributed between the microcircuit devices and conductor pads; providing the thermal path for heat that is generated by the switching functions; the cured adhesive supports and electrically insulates between interconnection particles and conductors on the microcircuits, and it prevents moisture or other contaminants from getting into or being entrapped within the interconnections.
Several problems arise from the use of isotropic conductive adhesives and anisotropic conductive adhesive that affect the capacity of the microcircuit, specifically, thermal dissipation and electrical interconnection. Those effects, in turn, can limit the number of circuit switches on, or logic operations performed by, a microcircuit. One of these problems is a need to apply high pressure to the microcircuit during fabrication that can damage or misalign parts of the circuit. Also, entrapped air in voids has lower thermal conductivity and can limit heat dissipation from the microcircuit. Third, increased resistance in the interconnect can result from insufficient interconnect particle to contact surface connection.
It would be advantageous to provide isotropic conductive adhesives and anisotropic conductive adhesive interconnects in which thermal and electrical interconnection resistance and distortion or damage of circuit boards are reduced or eliminated. With existing interconnects, regardless of specific metal or metallized polymer or carbonaceous material used for interconnects, or whether their surfaces are smooth or irregular, the thermal and electrical conductivity and board distortion or damage problems described above exist to varying degrees.